1. Field of the Invention
The present invention relates to a piezoelectric device including a plurality of piezoelectric resonators formed on the same substrate, an antenna duplexer, and a method of manufacturing the plurality of piezoelectric resonators.
2. Description of the Background Art
In recent years, with the expansion of mobile communications, devices for use in communication appliances have been expected to be small in size with high capability. One of the devices for mobile communication appliances is a filter, which is exemplarily structured by a piezoelectric resonator. This piezoelectric resonator is structured such that an upper electrode, a lower electrode, and a piezoelectric layer positioned between these electrodes are formed on a substrate. Also, the substrate is provided with a cavity or an acoustic mirror for ensuring vibrations of the piezoelectric resonator. In a ladder filter in which a plurality of such piezoelectric resonators are connected to form a ladder type, low-loss and high-attenuation characteristics are expected. Such a ladder filter is also applied to an antenna duplexer for mobile communication appliances.
To configure such a ladder filter and an antenna duplexer by using a plurality of piezoelectric resonators, a resonant frequency of each of the piezoelectric resonators has to be adjusted. For adjusting the resonant frequency of the piezoelectric resonator, the piezoelectric resonators are made different from each other in thickness of the piezoelectric layer, film thickness of the electrode, mass load, or the like. Examples of a technology of adjusting the resonant frequency of each piezoelectric element are disclosed in Japanese Patent Laid-Open Publication No. 2002-268645 (hereinafter referred to as a first conventional technical document) and Japanese Patent Laid-Open Publication No. 2002-359539 (hereinafter refereed to as a second conventional technical document).
The first conventional technical document discloses a technology for forming a plurality of piezoelectric resonators having different resonant frequencies by varying thicknesses of piezoelectric layers and film thicknesses of upper electrodes. FIG. 8 is a section view of a conventional piezoelectric device 800 with such piezoelectric resonators formed in this manner.
In FIG. 8, the piezoelectric device 800 includes piezoelectric resonators 810 and 820. The piezoelectric resonator 810 has a structure in which an acoustic mirror 811, a lower electrode 812, a piezoelectric layer 813, and an upper electrode 814 are sequentially formed on a substrate 801. The piezoelectric resonator 820 has a structure in which an acoustic mirror 821, a lower electrode 822, a piezoelectric layer 823, and an upper electrode 824 are sequentially formed on the substrate 801. The piezoelectric layer 813 is different from the piezoelectric layer 823 in film thickness. Also, the upper electrode 814 is different from the upper electrode 824 in film thickness. With this, the resonant frequencies of the piezoelectric resonators 810 and 820 are adjusted.
The second conventional technical document discloses a technology for forming a plurality of piezoelectric resonators having different resonant frequencies by varying mass loads on electrodes. FIG. 9 is a section view of a conventional piezoelectric device 900 with such piezoelectric resonators formed in this manner.
In FIG. 9, the piezoelectric device 900 includes piezoelectric resonators 910 and 920. The piezoelectric resonator 910 has a structure in which a cavity 911, a lower electrode 912, a piezoelectric layer 913, an upper electrode 914, and an oxidized conductive material portion 915 are sequentially formed on a substrate 901. The piezoelectric resonator 920 has a structure in which a cavity 921, a lower electrode 922, a piezoelectric layer 923, and an upper electrode 924 are sequentially formed on the substrate 901. The cavities 911 and 921 are provided to ensure vibrations of the piezoelectric resonators 910 and 920. As such, the oxidized conductive material portion 915 is formed so as to vary the mass load on the piezoelectric layer 913 from the mass load on the piezoelectric layer 923, thereby adjusting the resonant frequencies.
However, in the first conventional technical document, the film thicknesses of the piezoelectric layers have to be made different from each other in order to achieve desired resonant frequencies. This disadvantageously makes the manufacturing process complex.
Moreover, in the second conventional technical document, a material is added to a vibrating portion for increasing a mass load. However, this disadvantageously decreases an effective coupling coefficient of the piezoelectric resonators.